This invention relates to an aluminum-coated ferrous-base foil having a thickness not greater than about 0.133 mm (0.005 in) exhibiting improved oxidation resistance at elevated temperature and improved corrosion resistance in moist atmospheres containing water vapor and combustion gases, and to a method for making such foil. Although not so limited, the invention has particular utility in fabricated monolithic support structures in catalytic converters for exhaust systems of internal combustion engines. The largest market for such catalytic converters is in automotive pollution control systems. The invention includes further method steps carried out after making the foil which provide the foil with advantageous properties as a catalyst support structure or substrate, in addition to the oxidation and wet corrosion resistance properties of the foil.
A support structure or substrate for automotive-type pollution control catalysts requires elevated temperature oxidation resistance because the catalytic converter temperature can reach 1100.degree. C. (2000.degree. F.) for short periods of time under extreme operating conditions. The typical operating temperature range is from about 540.degree. to about 815.degree. C. (1000.degree. to 1500.degree. F.). Most steels can withstand only a few hours at 815.degree. C. in air or combustion gases before crumbling due to thermal oxidation. A catalyst support metal is required to maintain its structural integrity for at least 1000 hours at 815.degree. C. in an oxidizing atmosphere.
A support structure for automotive-type pollution control catalysts must also have wet corrosion resistance. Wet corrosion conditions occur when the exhaust system cools and condensate accumulates in the porous surfaces in the converter. Rusting must be avoided, primarily because the iron-containing corrosion products can combine with the active catalyst metal and destroy catalytic activity. As is well known, the active catalyst metals presently used for automotive pollution control are usually from the platinum group, such as platinum, rhodium and/or palladium.
Support structures of the above type further require a surface which will bond strongly to a heat resistant catalyst support material (such as gamma aluminum oxide, alkaline earth metal oxides, scandium oxide, and/or yttrium oxide) which is applied to the substrate in order to provide a large surface area for the active catalyst metal. Large gas volumes can be treated by a relatively small catalytic converter by using the increased surface area provided by a porous coating such as gamma aluminum oxide (typically called a washcoat). Cyclic thermal gradients cause spalling of the washcoat if it is not securely bonded to the substrate.
A support structure for automotive-type pollution control catalyst frequently has a honeycomb shape, and thin cell walls are required for this configuration. If the metal support material is formed from a continuous strip, it should be capable of reduction by rolling to foil thickness in order to meet the requirement for a thin cell wall. The thin cell walls exhibit three advantages. First, back pressure is reduced because there is less cross-sectional area to impede gass flow. Second, the catalyst begins working sooner because the lower mass of metal heats up faster. Catalytic converters must heat up to about 250.degree. C. (500.degree. F.) before conversion of combustion gases begins. Since the conversion reaction is exothermic, once the reaction starts the temperature will remain high enough to maintain the reaction until the flow of gases through the converter stops. The third advantage of a thin wall for honeycomb catalytic converters is the smaller cell size which is attainable. This smaller cell size increases the surface area-to-volume ratio, with consequent decrease in the size and cost of the converter.
Numerous prior art disclosures relate to metal catalytic converter substrates and to making ferrous base alloys for use in high temperature environments.
Published Japanese Patent Application No. 49-99982 discloses a catalyst support comprising a ferrous metal substrate, a porous iron-aluminum layer, and a porous aluminum oxide layer on which catalyst is deposited. The method comprises forming an aluminum layer on a foil by cladding, spraying, or hop dip coating, and heat treating at 700.degree. C. to 1300.degree. C. (1300.degree. F. to 2400.degree. F.) for 0.5 to 5 minutes to form a porous iron-aluminum layer. Preferably the heat treatment is conducted in an oxidizing atmosphere in order to convert the surface aluminum on the porous layer to aluminum oxide. The ferrous substrate can contain elements such as nickel, chromium and molybdenum. The heat treatment causes the aluminum in the coating and the metals in the substrate to "diffuse mutually." In a specific example an austenitic 18-8 stainless steel foil of 0.1 mm (0.004 in.) thickness was roughened and coated with molten aluminum with a coating thickness of 0.03 mm (0.0011 in.).
U.S. Pat. No. 3,059,326 discloses a method for making ferrous based alloys having substantial oxidation resistance and fortified for use in high temperature environments. The method involves the diffusion of an aluminum or aluminum alloy coating into a base metal containing from 3.5% to 8% aluminum by heating at 1300.degree. F. to 1600.degree. F. for one to three hours. The diffusion raises the aluminum content of the base metal to a total of about 16%. The alleged novelty resides in being able to carry out the desired working or cold reduction before coating since only slight working is possible after coating, according to the patentee. Coating thickness of 0.001 to 0.01 in. (0.025 to 0.25 mm) is disclosed.
U.S. Pat. No. 3,305,323 discloses the production of steel foil of 0.002 in. (0.05 mm) thickness or less, plated with tin, zinc, aluminum, alloys thereof and other metals. It is stated that already coated strip must be free of an intermediate iron-coating metal alloy layer in order to reduce the coated strip to foil thickness in proportion to the base metal during cold rolling. Ordinarily a reduction of 40% to 60% per pass is preferred. Diffusion of chromium and/or nickel coatings by heat treatment is suggested.
U.S. Pat. No. 4,079,157 discloses hot dip coating of an austenitic stainless steel with an aluminum-silicon alloy for automotive thermal reactors. It is stated that the use of pure aluminum coating results in a three-layer structure consisting of base material, which is essentially the unchanged austenitic stainless steel, an outermost layer which consists mainly of a ferritic iron-aluminum alloy, and a ferritic intermediate layer, which lies between the Fe-Al alloy layer and the base material. The different coefficients of thermal expansion of the ferrite and austenite layers cause stresses during cyclic heating with resulting plastic deformation of ferrite layers. The addition of silicon to the coating metal solved this problem since silicon (at 5% to 11%) forms an initial diffusion layer which inhibits subsequent formation of an aluminum diffusion layer. This in turn maintains the thickness of the ferrite layers within required limits, thereby avoiding plastic deformation.
U.S. Pat. No. 4,331,631 discloses a method of producing on the surface of a peeled foil of aluminum bearing ferritic stainless steel densely spaced aluminum oxide whiskers. The method consists of first forming a severely cold worked foil with an irregular surface by a metal peeling process. The foil contains 15% to 25% chromium, 3% to 6% aluminum, 0.3% to 1.0% yttrium (optional), and balance iron. The aluminum oxide wiskers are grown on the foil by heating the peeled foil in air at about 870.degree. C. to 970.degree. C. for a time sufficient to grow the oxide whiskers. The whiskers are stated to be about three microns high. The roughness of the whiskered surface substantially improves adhesion of an aluminum oxide washcoat and overcomes spalling problems encountered with oxide layers having typical smooth or nodular surfaces.
U.S. Pat. No. 4,318,828 discloses a method for forming aluminum oxide whiskers on the surface of an aluminum-containing ferritic stainless steel rolled foil. The method consists of a two part heat treatment. First, the foil is oxidized by heating in an atmosphere comprising predominantly an inert gas and containing 0.1 volume percent or less oxygen between about 875.degree. C. and 925.degree. C. (1606.degree. F. and 1700.degree. F.), said oxidation forming a surface-dulling film capable of producing dense whisker growth. Second, the foil is further oxidized by heating in air between about 870.degree. C. and 930.degree. C. (1600.degree. F. and 1780.degree. F.) for a time sufficient to grow densely spaced whiskers that substantially cover the surface. The method can be used to prepare a cold-rolled metal alloy foil containing 15% to 25% chromium, 3% to 6% aluminum, optionally 0.3 to 1.0 weight percent yttrium and the balance iron. The whiskers improve the adhesion of the aluminum oxide washcoat to the cold-rolled foil and thereby reduce spalling during converter use.
U.S. Pat. No. 4,188,309 discloses a shaped catalyst consisting essentially of a structural reinforcing agent of ferrous metal, a layer of a heat-resistant carrier material on the structural reinforcement agent, and a catalytically active component on the carrier material. The body of the structural reinforcing agent consists of cast or wrought iron, or carbon or low alloy steel and has a surface provided with a non-scaling, adhesive and anchoring-favoring aluminum/iron diffusion layer, this diffusion layer having been obtained by heating an aluminum-coated iron or steel at a temperature between 600.degree. C. and 1200.degree. C. (1100.degree. F. and 2200.degree. F.) for at least one minute.
U.S. Pat. No. 3,867,313 discloses an all metal, high temperature resistant catalyst element that consists of a base material comprised of primarily aluminum, chromium and iron on which is plated or deposited a noble metal comprising platinum and/or palladium. No aluminum oxide washcoat is used. The nickel-free, aluminum containing base material appears to be of advantage for at least certain all metal catalyst element operations and also results in substantially lower cost catalyst units.
Other patents of which applicant is aware, which show the general background of the art, include: U.S. Pat. Nos. 3,362,783; 4,096,095; 4,162,993; 4,277,374; 4,190,559; 3,873,472; 4,247,422; 3,920,583; 4,350,617; 3,907,708; 4,414,023.
Although the prior art is replete with disclosures relating to alloys and methods for making catalyst supports for catalytic converters, there is nevertheless a genuine need for a relatively low cost metal foil which combines high temperature oxidation resistance, wet corrosion resistance and surfaces that will bond securely to a porous aluminum oxide coating, and which can be readily formed from strip thickness material with conventional rolling mill equipment.